1. Field of the Invention
This invention relates generally to the field of metal matrix composites and, more particularly, to a method and apparatus for manufacturing micron-sized spherical alumina powders with polymorphic structure.
2. Background
Ceramic reinforced aluminum metal matrix composites (ALMMCs) offer unique combinations of properties. They can be engineered to have the stiffness of titanium, better wear resistance than steel and tailorable coefficient of thermal expansion, all while maintaining the lightweight characteristics of aluminum. Thus far, ALMMCs have been used primarily in certain aircraft and automotive components. More widespread use of ALMMCs has been impeded by two problems. One is the high cost associated with the subsequent machining of ALMMC materials with reinforcing particles such as SiC, B4C, Si3N4, and Ti2B after the basic material is fabricated. The second problem relates particularly to the use of alumina (Al2O3) particulates as the reinforcing agent. In this case, a significant problem arises during the installation of conventional Al2O3 powders in the aluminum alloy matrix of choice, i.e., from any of the 2000 series, 6000 series or 7000 series alloys. All of these matrix alloys contain important quantities of Mg as an alloy-strengthening agent. In order to create the ALMMC it is necessary to heat the material to temperatures well above room temperature. During the elevated temperature processing steps, conventional Al2O3 powders react with the Mg alloying agent in the matrix to form a spinel like compound. The chemical reaction removes the Mg as an active strengthening agent leading to an ALMMC that exhibits inferior mechanical properties.
It has been found that when micron-sized Al2O3 alloy powders whose particle shape is spherical and whose crystalline state is at least partially amorphous are incorporated into Al alloy matrices containing Mg as a strengthening agent, the chemical interaction between the reinforcing particles and Mg is fully suppressed, resulting in materials of very high strength. Once fabricated into a wrought product, this ALMMC material machines as easily as un-reinforced alloy material. This permits various engineered products to be made from this material in a very economical manner.
Current methods for manufacturing spherical alumina particles is to employ alloying elements such as magnesium, chromium or titanium, as disclosed in U.S. Pat. No. 6,814,903, or other elements such as boron, as disclosed in U.S. Pat. No. 5,340,781, to lower the melting point of Al2O3. Additives of alkali, halogens and alkaline earths and boron compounds greatly reduce the melting temperature of the alumina. These processes involve binding the alloying element with cation bonding in an aquious solution, drying the powder, milling the dried product and then firing to powder to form the spheres. This process is costly and time consuming and results in an impure Al2O3 final product.